The extinction of the Neanderthals could have been caused by their choosy appetites - they ate virtually nothing but meat, according to new a study.

"They were picky eaters," says Dr Paul Pettitt, at the University of Oxford, UK. "And this tells me that they are really unchanging - doing the same old thing year after year."

If their prey, such as bison and deer, then became scarce, they would struggle to survive.

Neanderthals lived in Europe between about 120,000 and 30,000 years ago. The cause of their extinction has been the subject of much debate and speculation has included their being killed off by early humans and their disappearing through interbreeding with humans.

"Excellent hunters"

"Neanderthals were excellent hunters," Dr Petitt told BBC News Online. "But the issue that was at stake was whether they hunted every day of their lives or whether it was just a summer outing."

Now new information, derived from remains found in Croatia, suggest that hunting was nearly all they did to gather food. This leads to the speculation that the more versatile diets of the early humans allowed them to survive when Neanderthals did not.

The early humans themselves may have been better hunters than the Neanderthals, depriving them of their kills. Or the hunted animals may have been struck by disease or migrated away.

It has been very hard to assess the variety of Neanderthal diets because although animal bones are often preserved in caves, easily rotted food like vegetables, fruit and grains rarely remain.

But the scientists found a way. They measured the ratios of the different types (isotopes) of carbon and nitrogen found in Neanderthal bones.

You are what you eat

Plants and animals have contrasting isotopic ratios, so when these are eaten they leave different signatures in a Neanderthal's bones. And because the bones grow slowly, the signature represents a 10 to 20-year average of the individual's diet, not merely the last meal.

They "calibrated" the analyses by comparing the Neanderthal bone ratios with those from contemporaneous animals at the top (bears) and bottom (bison) of the animal food chain.

The ratios showed that the Neanderthals were top-level predators, getting about 90% of their protein from meat. Previous research shows this sometimes included cannibalism. The rest of the protein would have come from nuts and grains.

Collagen in the jaw bone was analysed

The bones, a skull and a jaw, come from the Vindija cave site, north of the Croatian capital Zagreb. These are two of the youngest Neanderthal bones dated - just 28,000 years old and presumably coming from some of the last Neanderthals to exist.

Bones from bears, wolves, reindeer, and cattle or bison were also found in the cave and all showed signs of having been butchered for meat.

Isotopic analysis of Neanderthal bones has been done before in France, and produced the same ratios. But the previous study did not have the age dating or animal bone data to place it in context.

Another member of the scientific team, Fred Smith, from Northern Illinois University, US, said: "For several decades, archaeologists have debated the importance of meat in the Neanderthal diet, but this question never has been answered unequivocally.

"Our findings provide conclusive proof that European Neanderthals were top-level carnivores who lived on a diet of mainly hunted animal meat."

The research is published in the journal Proceedings of the National Academy of Sciences and was conducted by researchers from Oxford University, UK, Simon Fraser University, Canada, Washington University, St Louis, US, University of Bordeaux, France, Northern Illinois University, US, the Croatian Academy of Sciences and Arts and the University of Zagreb, Croatia.